Problem: $ A = \left[\begin{array}{rrrr}4 & 1 & -2 & 1 \\ 3 & 0 & 3 & 0\end{array}\right]$ What are the dimensions of matrix $ A$ ?
Explanation: The first dimension is the number of rows in the matrix. How many rows does $ A$ have? The second dimension is the number of columns in the matrix. How many columns does $ A$ have? So, $ A$ is a 2 $\times$ 4 matrix.